[petsc-users] KSPGMRESOrthog costs too much time

[Jed Brown]

On Thu, Nov 17, 2011 at 11:17, Rongliang Chen <rongliang.chan at gmail.com>wrote:

> In my log_summary output, I found that nearly 80% of the total time is
> spent on KSPGMRESOrthog. I think this does not make sense ( the log_summary
> output followed). Who has any idea about this?
>

Reductions are very expensive relative to everything else on the coarse
level. You can try more levels or a different coarse level solver. You can
also likely get away with solving the coarse problem inexactly.

Alternatively, you can try getting Chebychev to help you out. Use
-ksp_chebychev_estimate_eigenvalues to tune Chebychev (possibly to target a
specific part of the spectrum).

http://www.mcs.anl.gov/petsc/snapshots/petsc-dev/docs/manualpages/KSP/KSPChebychevSetEstimateEigenvalues.html


>
> Another question, I am using the two-level asm precondtioner.  On the
> coarse level I use one-level asm preconditioned GMRES to solve a coarse
> problem. So both the fine level solver and coarse level solver call the
> function KSPGMRESOrthog. In the log_summary output, I just know the total
> time spent on KSPGMRESOrthog and how can I know how much time is spent on
> the coarse level KSPGMRESOrthog and how much is spent on fine level
> KSPGMRESOrthog? Thanks.
>

I assume you are using PCMG for this, so you can add -pc_mg_log to profile
the time on each level independently. You seem to have many KSPGMRESOrthog
steps per fine-level PCApply, so I think most of the time is in the coarse
level.
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